1. Field of the Invention
The present invention relates generally to a system and method for controlling a braking force for an automotive vehicle which can improve a steering stability of the vehicle during a braking.
2. Description of the Background Art
A previously proposed braking force controlling system is exemplified by a Japanese Utility Model Registration Application No. Showa-59 155264.
In the disclosed braking force controlling system, a difference in pressure of braking between right and left tire wheels is used to control a vehicular yaw characteristic. Specifically, when the braking is applied, a steering angle of a vehicular driver exceeding a predetermined value, a timing at which the braking pressure for each of outer tire wheels with respect to a cornering force is increased is delayed so as to improve a cornering characteristic of the vehicle during the braking.
However, since, the previously proposed braking force controlling system does not take into consideration of the fact that a yaw rate generated due to a front wheel steering and a braking force difference between those at the right and left tire wheels is dependent upon a vehicle speed, it is difficult to control the yaw rate toward an appropriate value and it is difficult to improve a transient characteristic of the generated yaw rate.
A Japanese Patent Application No. Heisei-2 219867 (First Publication No. Heisei-3 281467 published on Dec. 12, 1991) exemplifies another driving force controlling system in which a target yaw rate is set on the basis of the vehicle speed and steering angle, a target right and left braking force difference required to make the target yaw rate coincide with an actually generated yaw rate is calculated on the basis of a vehicular model set according to the vehicular requirements and motion equations, and such a control that left and right tire wheel braking forces calculated according to the target left and right driving force difference coincides with actual left and right tire wheel braking forces.
Therefore, the transient characteristic of the yaw rate generated depending upon the vehicle speed can be improved. It is noted that the target left and right braking force difference is calculated using a cornering power of the vehicle tire wheels in the vehicular model.
However, since in the other braking force controlling system the increase in the braking pressure for one tire wheels is carried out and the decrease in the braking pressure for the other tire wheels is also carried out and a function of, so-called, anti-skid control so as to prevent the tire wheels from being locked is not provided.
Hence, in a case where the vehicle running on a road surface having a low motion friction coefficient (or slippery road surface) applies a braking and/or an abrupt (or full) braking, the tire wheels are locked so that the cornering force generated on the tire wheels becomes zero. Consequently, a large deviation occurs between the cornering power of the vehicular model and actually generated cornering power of the vehicle so that a controllability of the braking forces becomes reduced.